Priority is claimed to Patent Application No. 2001-16324 filed in Rep. of Korea on Mar. 28, 2001, herein incorporated by reference.
1. Field of the Invention
The present invention relates to a tunable optical oscillator, and more particularly, to a tunable optical oscillator that generates signals at a band of micro- to millimeter-wave.
2. Description of the Related Art
Increasing demands for the mobile Internet services including video and interactive services have resulted in the exhaustion of the available frequency band for wireless communications, and so triggered the development of the 4th generation wireless communication system. As the next generation communication system of the IMT-2000 (International Mobile Telecommunication), the micro/millimeter-wave communication that uses signals at the band of 3 GHz to 3000 Hz is drawing attention for broadband wireless communication.
However, because of its limitation on the transmission distance and need for high-frequency sources, the micro/millimeter-wave communication system requires to be hybrided with the existing wire systems. Low-attenuation, EMI-free optical fiber transmission is considered as the most promising candidate for long-haul transport of the high frequency band wireless signals, and the technology for generating optical micro/millimeter wave is the key technology for the broadband communication system.
Among the system design options for the hybrid optical wireless system, generating and transmitting optical millimeter wave signals has many advantages. While the baseband transmission has a disadvantage of highly complex outdoor base stations with full SDH/SONET compliant equipments, the optical mm-wave transmission system has relatively simple outdoor base stations with compact remote antenna units. Thus, it can reduce loads of many antennas in pico-cell communication networks Moreover, as the high-frequency optical signals are generated at a central station it can be built a centralized system. With control functions at the central station, the channel allocation, hand-over and antenna can be easily controlled, and the number of subscriber that the system can support increases. In addition to the advantage of easy system maintenance, it has transparency to modulation types. Since the generated optical signals are modulated by electric baseband signals, it is flexible to any type of modulation formats. With these advantages, it enables cost-effective and efficient communication and provides flexibility to radio access in the subscriber loop such as mobile LAN or broadband wireless local loop (B-WLL) systems.
In the hybrid fiber radio system, methods for generating optical micro/millimeter-wave signals can be classified according to modulation methods or the number of the light sources.
The conventional methods for generating micro/millimeter-wave signals include a direct modulation method (P. A. Morton, Electron. Lett., Vol. 30, No. 24, p. 2044, 1994), and an external modulation method (U. Gliese, IEEE Trans. Microwave Theory Tech., Vol. 44, No. 10, p. 1716, 1996). However, these methods are costly due to the complexity and cost of electronic circuits and have limitation on the achievable frequency. The methods using one light source include a sideband techniques (G. H. Smith, IEEE Trans. Microwave Theory Tech., Vol. 45, No. 8, p. 1410, 1997), a mode locking laser (T. Kuri, IEEE Trans. Microwave Theory Tech., Vol. 47, No. 5, p. 570, 1999) and a dual mode laser method (D. Wake, IEEE Trans. Microwave Theory Tech., Vol. 43, No. 9, p. 2270, 1995). These methods are not practical, either, in that manufacturing and developing these optical devices is difficult and costly.
In consideration of cost, frequency tunability, and upgradability to multi-channel systems, it is desirable to adopt a multi-source method, that is, a heterodyne method that uses beating between signals from different sources to produce signals of the wanted frequency.
One of the heterodyne methods using injection locking was introduced by L Noel (IEEE Trans. Microwave Theory Tech., Vol. 45, No. 8, p. 1416, 1997). In this method, a continuous wave light from a master laser is injected into a RF-modulated slave laser and locks one of sideband modes of the slave laser. Then, the optical signals of tens of GHz, which corresponds to the difference between the lasing and injected frequencies of the slave laser, was generated by beating between two signals. However, since the power of the sideband modes are relatively low and so the range of the available sideband modes is limited, the maximum achievable frequency with this scheme is limited. It depends on the modulation response of the master and slave lasers.
To solve the above problems, it is an objective of the present invention to provide an apparatus for generating optical micro/millimeter wave signals with tunability by injection locking of four-wave mixing (FWM) conjugates modes.
To achieve the above objective, there is provided a tunable optical oscillator comprising an electrical signal generator for generating electrical signals at an RF band; a master laser driven by the electrical signals and for generating optical signals of a first frequency; and a slave laser driven by the electrical signals, and for generating optical signals of a second frequency; the apparatus that feeds the signals from the master laser to the slave laser to carry out the four-wave-mixing and injection-locking, and that outputs the resulting signals from the slave laser.